Takenori Oida

Magnetic resonance elastography: in vivo measurements of elasticity for human tissue

Elasticity is an important physical property of material. In the clinical practice, elasticity is used for physical examination in several ways, such as palpation or percussion. Differences in elasticity can help facilitate the diagnosis of tumors and their extent. Elasticity is an essential property in the diagnosis of liver cirrhosis, or soft degeneration in tissue necrosis. In addition, information of tissue elasticity is utilized in virtual reality systems such as telepalpation and computer assisted surgery. It was difficult to obtain such properties in vivo by using conventional measurement methods. To overcome this problem, magnetic resonance elastography (MRE) has been developed that provides noninvasive in vivo measurements of elasticity for human tissue. We summarize this MRE method in this paper. When an object is oscillated from the surface in a known frequency, acoustic strain waves propagate into the material and one can calculate the physical constants of a material elasticity by the wave velocity. In MRE measurements, a cyclic micromotion caused by the acoustic strain waves is obtained as an MR image that is synchronized to the oscillation. By measuring the local wavelength of the strain waves, we can obtain the elasticity constants. Several examples of MRE image including in vivo measurements are provided as well as several methods to estimate the local wavelength from MRE images are described.

Detecting rotating magnetic fields using optically pumped atomic magnetometers for measuring ultra-low-field magnetic resonance signals

In this paper, we describe the detection of rotating magnetic fields using optically pumped atomic magnetometers (OPAMs) for measuring magnetic resonance (MR) signals. From the results of rotating- and alternating-magnetic-field measurements, we found that to detect a rotating magnetic field with high sensitivity, the rotation direction of the magnetic field to be measured must select the bias-magnetic-field direction of OPAM. In addition, the OPAM sensitivity for rotating magnetic fields should be twice that for alternating magnetic fields. These results indicate that for measuring MR signals, magnetic fields caused by rotating magnetizations can be detected with the sensitivity of 10 fT(rms)/Hz order at 1 kHz using OPAMs.

Direct Detection of Magnetic Resonance Signals in Ultra-Low Field MRI Using Optically Pumped Atomic Magnetometer With Ferrite Shields: Magnetic Field Analysis and Simulation Studies

An ultra-low field (ULF) magnetic resonance imaging (MRI) system with an optically pumped atomic magnetometer (OPAM) has recently been proposed. However, to measure MR signals with high sensitivity in ULF-MRI systems with OPAMs, the resonant frequencies of a sample and an OPAM must be matched. In this study, we propose a direct detection method of the MR signals using ferrite shields. In addition, the magnetic field distribution analyses and MR signal intensity simulations were performed to improve the sensitivity of direct MR signal detection by using an OPAM with ferrite shields. The magnetic field distribution analyses and MR signal intensity simulations indicated that the MR signals could be detected by the proposed method and that the ferrite shields with high relative permeability improved the sensitivity of direct MR signal detection.

Optimization of Flux Transformer for Optically Pumped Atomic Magnetometer in Ultra-Low Field MRI Systems

An ultra-low field (ULF) magnetic resonance imaging (MRI) system with an optically pumped atomic magnetometer (AM) has recently been proposed. Because AM does not require cryogenic cooling, it can easily measure extremely small magnetic fields. However, to measure magnetic resonance (MR) signals with high sensitivity in ULF-MRI systems with AMs, the resonant frequencies of the sample and alkali metal in the AM must be same. To satisfy this requirement, a flux transformer (FT) has been proposed to detect MR signals. In this study, the simulations in the output coil of the FT and pseudo-MR signal measurements were performed to improve the signal-to-noise ratio (SNR) in the remote detection of MR signals by using AM with FT. The simulations and measurement results indicate that, to improve the SNR of the detector, the output coil of the FT should be placed in the vicinity of a glass cell, and the number of turns and radius of the output coil need to be optimized.

Actively Shielded Bias Field Tuning Coil for Optically Pumped Atomic Magnetometer Toward Ultralow Field MRI

In this paper, we describe a method for directly detecting magnetic resonance (MR) signals using optically pumped atomic magnetometers (OPAMs) in ultra-low-field MR imaging (ULF-MRI). For direct detection with OPAMs in ULF-MRI, we propose the use of an actively shielded bias field tuning coil to match the resonant frequencies between MR signals and OPAM. Design of the tuning coil was based on both a target field approach and a stream function method. The results of field distribution analyses demonstrated that the resonant frequency of an OPAM could be tuned by the actively shielded bias field tuning coil. In addition, the actively shielded bias field tuning coil minimized the distortion of an external magnetic field in the sample region. These results indicate that actively shielded bias field tuning coil can be designed to match the resonant frequencies of MR signal and an OPAM with the inhomogeneity of sample region <;0.25%. However, the actively shielded bias field tuning coil must be fabricated with an error of 0.5 mm or less between the centers of the primary and shield coils.

Thermal noise calculation method for precise estimation of the signal-to-noise ratio of ultra-low-field MRI with an atomic magnetometer.

In recent years, there has been considerable interest in developing an ultra-low-field magnetic resonance imaging (ULF-MRI) system using an optically pumped atomic magnetometer (OPAM). However, a precise estimation of the signal-to-noise ratio (SNR) of ULF-MRI has not been carried out. Conventionally, to calculate the SNR of an MR image, thermal noise, also called Nyquist noise, has been estimated by considering a resistor that is electrically equivalent to a biological-conductive sample and is connected in series to a pickup coil. However, this method has major limitations in that the receiver has to be a coil and that it cannot be applied directly to a system using OPAM. In this paper, we propose a method to estimate the thermal noise of an MRI system using OPAM. We calculate the thermal noise from the variance of the magnetic sensor output produced by current-dipole moments that simulate thermally fluctuating current sources in a biological sample. We assume that the random magnitude of the current dipole in each volume element of the biological sample is described by the Maxwell-Boltzmann distribution. The sensor output produced by each current-dipole moment is calculated either by an analytical formula or a numerical method based on the boundary element method. We validate the proposed method by comparing our results with those obtained by conventional methods that consider resistors connected in series to a pickup coil using single-layered sphere, multi-layered sphere, and realistic head models. Finally, we apply the proposed method to the ULF-MRI model using OPAM as the receiver with multi-layered sphere and realistic head models and estimate their SNR.

Construction of human head voxel models from MR images for EEG analysis based on EM algorithm

In order to enhance the reliability of electroen-cephalogram (EEG) signal source analyses, utilizing EEG lead field matrices obtained by field analyses in custom-made real head models is effective technique. Custom-made models are usually constructed from voxel models derived from magnetic resonance (MR) images using a variety of image-processing techniques. We have improved one of the techniques that select threshold levels dividing signals and noises based on the EM (expectation-maximization) algorithm. This technique contributes to rapid and high-quality voxel model acquisition. We demonstrate the following operations: (a) voxel model construction, (b) lead field calculation, and (c) simulation of EEG electrode voltage measurement induced by an equivalent current dipole (ECD), which is set in the primary motor cortex by considering application of brain-machine interfaces. The proposed technique is compared with other techniques based on the voltage differences caused by the constructed model-shape differences.

An MR-DTI-based fiber tracking method for the multimodal integrative study of cognitive brain functions

We have developed a method for white matter fiber tracking inferred from magnetic resonance diffusion tensor imaging (MR-DTI) to match functional and anatomical information in the human brain. Functional information was obtained using functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG). One of the major problems in fiber tracking based on MR-DTI is the error of fiber orientation estimation at areas of fiber crossing. Here, we propose a novel fiber tracking method involving searching for similarity of direction vectors in the vicinity of fiber crossing areas. The proposed method was tested on simulation images and its utility was validated. Subsequently, the method was applied to in vivo data, and we tried to fuse the data obtained with the method with functional information obtained from a newly developed fMRI-MEG integrative method. The fMRI-MEG integrative method successfully detected dynamic cortical activities in multiple visual areas, such as V1, V2, and V5, during an apparent visual motion perception task. Our fiber tracking results demonstrate that the present method can be used to confirm connectivity among multiple cortical areas associated with higher cognitive brain functions.

Remote detected Low-Field MRI using an optically pumped atomic magnetometer combined with a liquid cooled pre-polarization coil

Superconducting quantum interference devices are widely used in basic and clinical biomagnetic measurements such as low-field magnetic resonance imaging and magnetoencephalography primarily because they exhibit high sensitivity at low frequencies and have a wide bandwidth. The main disadvantage of these devices is that they require cryogenic coolants, which are rather expensive and not easily available. Meanwhile, with the advances in laser technology in the past few years, optically pumped atomic magnetometers (OPAMs) have been shown to be a good alternative as they can have adequate noise levels and are several millimeters in size, which makes them significantly easier to use. In this study, we used an OPAM module operating at a Larmor frequency of 5kHz to acquire NMR and MRI signals. This study presents these initial results as well as our initial attempts at imaging using this OPAM module. In addition, we have designed a liquid-cooled pre-polarizing coil that reduces the measurement time significantly.

Non-Invasive Measurement of In-Vivo Elasticity of Skeletal Muscles with MR-Elastography

In order to evaluate the mechanical properties of the human skeletal muscles, the elasticity and viscosity of the human calf muscles were measured with Magnetic Resonance Elastography (MRE). MRE is a novel method to measure the mechanical properties of living soft tissues in vivo quantitatively by observing the strain waves propagated in the object. In this study, the shear modulus and viscosity coefficient were measured with MRE. The shear modulus was 3.7 kPa in relaxed state, and increased with increasing the muscle forces. Interestingly, the viscosity was changed with the vibration frequency applied to the muscles, that was 4.5 Pa·s at 100Hz vibration and 2.4 Pa·s at 200Hz vibration. This shows clearly the visco-elastic property.